1. Field of the Invention
The present invention relates to linear beam devices used for amplifying a radio frequency (RF) signal, such as inductive output tubes. More particularly, the invention relates to an inductive output tube having a multi-staged depressed collector configured to achieve improved efficiency.
2. Description of Related Art
It is well known in the art to utilize a linear beam device, such as a klystron or traveling wave tube amplifier, to generate or amplify a high frequency RF signal. Such devices generally include an electron emitting cathode and an anode spaced therefrom. The anode includes a central aperture, and by applying a high voltage potential between the cathode and anode, electrons may be drawn from the cathode surface and directed into a high power beam that passes through the anode aperture. One class of linear beam device, referred to as an inductive output tube (IOT), further includes a grid disposed in the inter-electrode region defined between the cathode and anode. The electron beam may thus be density modulated by applying an RF signal to the grid relative to the cathode. After the anode accelerates the density-modulated beam, the beam propagates across a gap provided downstream within the IOT and RF fields are thereby induced into a cavity coupled to the gap. The RF fields may then be extracted from the output cavity in the form of a high power, modulated RF signal.
At the end of its travel through the linear beam device, the electron beam is deposited into a collector or beam dump that effectively captures the remaining energy of the spent electron beam. The electrons that exit the drift tube of the linear beam device are captured by the collector and returned to the positive terminal of the cathode voltage source. Much of the remaining energy of the electrons is released in the form of heat when the particles strike a stationary element, such as the walls of the collector. This heat loss constitutes an inefficiency of the linear beam device, and as a result, various methods of improving this efficiency have been proposed.
One such method is to operate the collector at a xe2x80x9cdepressedxe2x80x9d potential relative to the body of the linear beam device. In a typical linear beam device, the body of the device is at ground potential and the cathode potential is negative with respect to the body. The collector voltage is depressed by applying a potential that is between the cathode potential and ground. By operating the collector at a depressed potential, the opposing or decelerating electric field within the collector slows the moving electrons so that they can be collected at reduced velocities. This method increases the electrical efficiency of the linear beam device as well as reducing undesirable heat generation within the collector.
It is also known for the depressed collector to be provided with a plurality of electrodes arranged in sequential stages in a structure referred to as a multi-staged depressed collector. Electrons exiting the drift tube of the linear beam device actually have varying velocities, and as a result, the electrons have varying energy levels. To accommodate the differing electron energy levels, the respective electrode stages have incrementally increasing negative potentials applied thereto with respect to the linear device body, such that an electrode having the highest negative potential is disposed the farthest distance from the interaction structure. This way, electrons having the highest relative energy level will travel the farthest distance into the collector before being collected on a final one of the depressed collector electrodes. Conversely, electrons having the lowest relative energy level will be collected on a first one of the depressed collector electrodes. By providing a plurality of electrodes of different potential levels, each electron can be collected on a corresponding electrode that most closely approximates the electron""s particular energy level. Thus, efficient collection of the electrons can be achieved.
As disclosed in U.S. Pat. No. 5,650,751, a substantial improvement in efficiency of an IOT can be realized by operating the device with a multi-staged depressed collector. When the IOT is configured such that beam current passes through the IOT during a portion of a full cycle of the RF input signal, both the DC current and collection voltage would go up and down with the RF output voltage, and both would be proportional to the RF output voltage or the square root of the output power. In other words, the input power would be proportional to the output power at all power levels, thereby providing very nearly constant efficiency across the operating range of the device with a proper choice of collector electrode voltages. An IOT having a multi-stage depressed collector is therefore referred to herein as a constant efficiency amplifier (CEA). The aforementioned U.S. Pat. No. 5,650,751 is incorporated by reference herein in its entirety.
Accordingly, it would be desirable to further improve the efficiency achieved by a constant efficiency amplifier.
The present invention satisfies the need for an inductive output tube (IOT) having a multi-staged depressed collector that provides further improvements in efficiency. In accordance with the teachings of the present invention, an IOT having a multi-stage depressed collector is referred to herein as a constant efficiency amplifier (CEA).
In a first embodiment, a CEA is provided with an electron gun and has a tube body. The electron gun generates an electron beam. The electron beam travels through the tube body. The CEA is also provided with a magnetic solenoid that produces a magnetic flux that focuses the electron beam as it travels through the tube body. The magnetic flux includes a portion that threads through the electron gun. The CEA is adapted to reduce this portion of the magnetic flux in order to further improve the efficiency achieved by the CEA.
In a second embodiment, an amplifying apparatus is provided with an electron gun. The electron gun has a cathode, an anode, and a grid disposed between the cathode and anode. The anode is spaced a distance away from the cathode. The cathode provides an electron beam that passes through the grid and the anode. The grid is coupled to an input radio frequency signal that density modulates the electron beam. The amplifying apparatus is also provided with a drift tube that is spaced away from the electron gun. The drift tube surrounds the electron beam (produced by the electron gun) and contains a first portion and a second portion. A gap is defined between the first and second portions. A polepiece is connected with the drift tube and holds the first portion in an axial position relative to the cathode and the grid. The polepiece also has a first side facing the cathode and a second side facing away from the cathode. The amplifying apparatus is further provided with an output cavity coupled with the drift tube. The density modulated electron beam passes across the gap and couples an amplified radio frequency signal into the output cavity. The amplifying apparatus also contains a depressed collector spaced away from the drift tube. The electron beam passes into the collector after transit across the gap. The collector has a plurality of electrode stages. Each of the stages is adapted to have a respective electric potential applied to it.
A first magnetic solenoid is located on the second side of the polepiece. The first magnetic solenoid generates a magnetic flux line. The magnetic flux line guides the electron beam as it passes through the gap. A portion of the magnetic flux line threads through the cathode. A second magnetic solenoid is located on the first side of the polepiece and produces a magnetic field that effectively cancels the portion of the magnetic flux line that threads through the cathode. Alternatively, the polepiece may have a hole extending through the polepiece in the axial position relative to the cathode and the grid. The diameter of the hole is dimensioned to reduce the portion of the magnetic flux line that threads through the cathode.
In addition, the plurality of electrodes stages may include a first electrode stage and a plurality of remainder electrode stages. In one embodiment, the plurality of remainder electrode stages include a last stage. The last stage has an inner length and a minimum inner diameter. The inner length is at least twice the minimum inner diameter. In another embodiment, the plurality of remainder electrode stages include at least two stages that are connected together electrically. The two stages of the plurality of remainder electrode stages include a total inner length and a minimum inner diameter. The total inner length exceeds twice the minimum inner diameter. In an alternate embodiment, the plurality of remainder electrode stages include a last stage and a penultimate stage. The last stage is connected to a potential slightly higher than that of the penultimate stage.
A more complete understanding of the present invention will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the embodiment. Reference will be made to the appended sheets of drawings, which first will be described briefly.